Abstract
Oxide facies gold deposits are characterised by high Au-tenor free-milling gold nuggets and differ to their associated sulphide assemblages which have a lower Au-tenor. These changes point to an active gold geochemical cycle during the transitioning from sulphide- to oxide facies, which commonly also correlates with the formation of an iron oxide-rich gossan. Surficial gold occurrences in the Amani area of southwestern Tanzania are hosted by gossanous outcrops comprising weathered- to highly weathered quartz-carbonate veins and high Au-tenor gold, and thus represent an ideal natural laboratory in which to study supergene gold enrichment processes. The gossan mineralogy comprises goethite, hematite, malachite, azurite, siderite and quartz assemblages, with supergene gold particles up to 6 mm and with an average gold concentration of 95 wt%. The supergene gold particles have irregular and elongate morphologies, with elongate gold occurring exclusively along narrow ribs of neoformed goethite. The crystal habit of this gold shows a persistent orthogonal relationship between adjacent sub-micrometre-sized terraces, reflecting a possible crystallographic control on supergene gold precipitation. Novel results from 3-D X-ray computed tomography reveal that the micrometre- to millimetre scale distribution of gold follows a near-orthogonal pattern controlled by the textures and fluid flow pathways within the developing gossan. These elongate gold particles form interconnected 3-D skeletal gold networks which coalesce at discrete intersections to form larger nuggets (up to 6 mm in length) of high Au-tenor supergene gold. A detailed geochemical consideration of the mineralogy of the gossan, its precursor vein paragenesis and the surrounding host rocks suggest that the physicochemical conditions within the developing gossan favour supergene gold growth by dissolution-re-precipitation mechanisms involving an intermediate gold thiosulphate ligand complex.
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